That is true for everyone excluding the princesses but just because darkness doesn't reside in peoples hearts doesn't mean everyone's heart is pure light either, what I mean by this is that just because you have never done anything bad it doesn't necessarily make you a saint because you might not have done much good stuff either. The princesses could still be the only people with hearts of pure light, even if there is no darkness in everyone's hearts yet.
It's all debatable though, and only time will truly tell, although I am sticking to my theory about Unbirths being pure living beings of darkness for now, unless some new information discredits it completely.